Design principles for ultra-low humidity sensors: a mechanism-oriented review

Abstract

The precise detection of ultra-low relative humidity (<10% RH) is essential in semiconductor manufacturing, lithium-ion battery production, and aerospace systems, where even trace amounts of moisture can lead to irreversible performance degradation. In contrast to previous reviews that concentrated on materials or transduction methods, this review offers a mechanism-oriented analysis of humidity sensing in dry environments. It explores how underlying physical principles influence sensor performance and guide material design for industrial applications. We conduct a systematic analysis of how chemisorption, dielectric modulation, quantum tunnelling, and interfacial charge transfer influence sensor performance at sub-10% RH, comparing these mechanisms with the conventional physisorption/ionic conduction that dominates at higher humidity levels. Significant advancements in material design such as defect-engineered metal oxides, hydrophilic metal–organic frameworks (MOFs), and two-dimensional heterostructures are assessed for their potential in improving sensitivity while reducing hysteresis, drift, and slow response kinetics. Innovative approaches like hybrid optical-electronic sensing and AI-driven calibration are emphasized as solutions to address the longstanding trade-offs between sensitivity and stability. By linking fundamental physical mechanisms with materials design, this review aims to guide the development of next-generation humidity sensors that combine high sensitivity, low hysteresis, and long-term stability, even under ultra-dry conditions.